1. Field of the Invention
The present invention relates to a plasma welding apparatus for a guide thimble and guide thimble end plug of a nuclear fuel assembly, capable of improving productivity in welding the guide thimbles and their end plugs constituting the nuclear fuel assembly.
2. Description of the Related Art
Nuclear reactors are facilities for artificially controlling a fission chain reaction of a fissionable material in order to use thermal energy generated from nuclear fission as power.
Nuclear fuel used in the nuclear reactor is manufactured by forming concentrated uranium into uniformly sized cylindrical pellets (hereinafter referred to as “sintered compacts”) and then charging a plurality of sintered compacts in a fuel rod. This plurality of fuel rods constitutes a nuclear fuel assembly, wherein the fuel rods are charged in a reactor core, and are then burnt by a nuclear reaction.
The fuel rod assembly includes spacer grids 1 into which fuel rods are inserted, a plurality of guide thimbles (or guide tubes) 2 fixed to the spacer grids 1, an upper end fitting (or top nozzle) 3 fastened to upper ends of the guide thimbles 2, and a lower end fitting (or bottom nozzle) 4 fastened to lower ends of the guide thimbles 2. Each fuel rod is supported by dimples and springs formed on each spacer grid 1.
Particularly, each guide thimble is coupled to the spacer grid and the upper and lower end fittings so as to form a framework of the nuclear fuel assembly, and supports a load of the fuel rods within the nuclear fuel assembly. Further, each guide thimble functions as a guide channel that guides insertion of a control rod controlling output of the nuclear reactor, as well as a passage channel for a poison rod or a neutron source rod. As a material of the guide thimble, a zirconium alloy is used like a cladding tube of the fuel rod.
The guide thimble is configured so that an open lower end thereof is closed by a lower end plug. Thus, the control rod inserted into the guide thimble from the top is prevented from falling down by the lower end plug of the guide thimble.
Since the guide thimble plays a considerably important role in the nuclear reactor, it should be manufactured with high precision. Particularly, a process of coupling the end plug requires very precise work in order to secure quality. Further, the process takes a very long time when it is manually carried out.
Further, the guide thimble has a length of about four meters, and is difficult to handle. Furthermore, the guide thimble does not permit re-welding. When the guide thimble and its end plug are manually welded, this provides very low productivity, and there is a difficulty in securing a high quality.
To automatize the process of welding the guide thimble and the end plug, the applicant of the present invention has proposed an automated welding apparatus and method for the guide thimble plug of the guide thimble in the nuclear fuel assembly, which is disclosed in Korean Patent Nos. 10-0775577 and 10-0775578 (issued on Nov. 5, 2007). These patents are designed to automatically supply the guide thimbles and their end plugs to a welding chamber so as to automatize welding work in the welding chamber.